1. Field of the Invention
The invention relates to a device and a method for the optical selection of components of a least one fraction from a bulk material stream conveyed along a conveyance direction, having a camera unit, whose viewing direction is oriented toward the bulk material stream, a background, which is situated in the camera viewing direction behind the bulk material stream, and whose color is adaptable to the color of a selected fraction of components of the bulk material stream, an analysis and control unit, which is connected to the camera unit, and in which control commands for a separation unit, which is capable of separating the components to be selected out of the bulk material stream upon activation, may be generated according to a decision criterion.
2. Description of the Prior Art
In known facilities for the automatic optical sorting of bulk materials, the material to be sorted is applied in a single-level layer on a conveyor belt as much as possible. The conveyor belt runs at a velocity of 3 m/s, for example. At the end of the conveyor belt, the material is thrown off from the conveyor belt and flies further in a trajectory. Shortly after the ejection edge, a line-scan camera looks at the material stream. The images recorded by the camera are analyzed by a computer. The components of the bulk material stream which are to be sorted out are recognized on the basis of their color and possibly also the shape which are blown out of the free-flying material stream with the aid of short compressed air blasts. In other implementations of optical sorters, the free-flying material stream is not generated via a conveyor belt, but rather the bulk material stream slips over a chute or the bulk material stream is poured in the form of a freefalling material stream.
An important detail in the design of optical sorting devices of this type relates to the optical background, in front of which the camera records the material to be sorted. The background is typically designed in such a way that the objects to be selected and thus to be recognized stand out with sufficient contrast from the background in each case.
The first choice is a passive black background in many cases, which may easily be implemented as a light trap. If deep-black objects must also be recognized in the material stream, a homogeneous active background is selected instead, which is lite up in a color which does not occur in the bulk material—for example, a pure blue. However, this procedure is only advisable if the objects to be recognized are sufficiently large in comparison to the local resolution of the imaging optics and a camera and correspondingly sufficiently many pixels are available for recognizing color and shape of the object. In contrast, if the objects are too small, their color may no longer be reliably recognized. To solve this problem, it is fundamentally possible to increase the local resolution of the system. However, the material throughput and thus the economic utility are thus decreased at a given system performance.
In some known applications, the color of the background is selected in accordance with the color of the material to be let through, so that the material to be let through optically disappears in front of the background because of the color equalization thus provided and only the objects to be blown out stand out in front of the background with sufficient contrast. For example, if small objects of other colors are to be recognized in a material stream of small red objects, a red background would be selected, before which only the non-red objects stand out, which may then be registered by the camera unit.
The design of an adaptive background is entirely advantageous and already proven in practice. However, the difficulty results from implementing a homogeneous background in the desired color and brightness. This is true in particular if different materials are sorted on a sorting machine in relatively short chronological succession and corresponding work for changing over the background becomes necessary with each material change.
In some applications of the optical sorting of bulk materials, the objects may already be classified on the basis of their grayscale, that is, on the basis of their brightness. An analysis of the color is not necessary in these cases. In such cases, a settable background illumination is used in a known way. For the implementation, for example, a diffusely reflecting surface may be applied in the background, which is irradiated by one or more fluorescent tubes. The brightness of the background is then settable to the desired value by selection of a surface having a suitable remission factor and by regulating the lamp brightness. The local curve of the brightness over the sorting width may be adapted individually as needed, for example, by suitable screens in front of the lamps.
For the formation of a color background having a defined color, in the prior art, one or more white lamps are used and the surface applied in the background is given a suitable color. For this purpose, for example, colored adhesive films are used or the desired color tone is mixed as a colored lacquer. This method is used in sorting machines to recognize foreign bodies in tobacco, for example.
A further method according to the prior art comprises using the test subjects themselves for the production of the background color, in that a representative sample is ground and the desired color is produced from the ground material, for example.
In all of the above cases, the setting of background lighting in the color of the material flow to be let through is very complex and is only worth it if the same product is sorted on the relevant sorting machine over a longer period of time. Further disadvantages of the configurations used until now are the susceptibility of the background to contamination, fading of the color, or mismatching as a result of lamp aging. Moreover, it is only possible to produce the desired brightness curve over the entire sorting width with quite high effort.
Reference is made to the following publications in regard to the published, known prior art:
A channel sorter is described in EP 0 146 299 B1, in which a bulk material stream comprising coffee beans, for example, falls through a measuring cell. The measuring cell is composed, on the one hand, of an illuminated background and, on the other hand, a detector which has discrete photo sensors. The bulk material stream falls perpendicularly through the so-called observation zone, which is delimited by the detector having a viewing direction toward the background. If a flawed part, which stands out in color from the background, is detected in the bulk material stream, an activation of a high-pressure nozzle device, which is downstream from the measuring cell in the falling direction of the bulk material stream, is performed to select the flawed part out of the bulk material stream.
A method for the selection of flawed parts from a bulk material stream, which is based on a comparable selection principle as the previously described method, may be inferred from US 2003/0034282 A1. In this case the bulk material stream is registered by a camera unit in front of an LCD display, which represents the color background. To avoid color drifts of the color background, the color impression is registered using a spectrometer and corrected appropriately in the event of deviations.
US 2006/0016735 A1 describes a sorter for transparent granules, which fall from a belt conveyor along a trajectory through two detector units situated along the ejection path, of which one detector unit registers the front side and the other detector unit registers the back side of the granules. By comparing the registered front and back recordings, the transparency of the components may be concluded and, furthermore, flawed parts detected on the basis of a selection criterion may be sorted out of the bulk material stream using a known air pressure nozzle configuration.